They say Mr. Romney lacks trust on issues such as outlawing abortion and opposing same-sex marriage and because he is a Mormon. Opposition is particularly powerful among those who supported former Arkansas Gov. Mike Huckabee in the Republican presidential primaries earlier this year.

"McCain and Romney would be like oil and water," said evangelical novelist Tim LaHaye, who supported Mr. Huckabee. "We aren't against Mormonism, but Romney is not a thoroughgoing evangelical and his flip-flopping on issues is understandable in a liberal state like Massachusetts, but our people won't understand that."

The Rev. Rob McCoy, pastor of Calvary Chapel in Thousand Oaks, Calif., who speaks at evangelical events across the country, told The Washington Times, "I will vote for McCain unless he does one thing. You know what that is? If he puts Romney on the ticket as veep.

"It will alienate the entire evangelical community - 62 million self-professing evangelicals in this country, half of them registered to vote, are going to be deeply saddened," Mr. McCoy added.

Mr. Huckabee, an ordained Southern Baptist minister, was the favorite of evangelical voters in the Republican presidential nomination contest earlier this year and won more delegates per dollar spent than any other candidate in either party.

Other well-placed Christian conservatives say that although many evangelical leaders could accept and work for a McCain-Romney ticket, Mr. Huckabee's supporters tend to be "rabid" in their views against Mr. Romney because of his faith: They do not regard Mormonism as a Christian denomination.

The McCain campaign will say officially only that the choice hasn't been made and that the wealthy former governor of Massachusetts is just one of several options for the Republican ticket.

In conversations with The Times, several Republican officials close to the McCain campaign also played down anti-Romney sentiment among conservative evangelicals. They cited an online poll of evangelicals by 2000 presidential primary candidate Gary Bauer that found Mr. Romney is the top vice-presidential choice of born-again Christians.

But in an interview with The Times, Mr. Bauer, who was a domestic policy adviser in the Reagan administration, described the results of his poll as more ambiguous than that.

"In our online poll, Romney won a plurality, and Mike Huckabee ran a strong second," said Mr. Bauer, who also told The Times that he does not think Mr. Romney ought to be a drag on the ticket. "But a lot of the Huckabee supporters said if Romney is McCain's choice, they would bail out in November."

An evangelical leader who, though he has close ties to Mr. McCain, confided to The Times that polling suggests that putting Mr. Romney on the ticket likely would cost Mr. McCain 7 percent to 10 percent of the evangelical vote - enough to spell defeat for Mr. McCain in a close race with Sen. Barack Obama, the presumptive Democratic nominee.

White evangelicals have become the Republican bedrock in recent elections, providing President Bush with 35 percent of his votes in 2004. Even in 2006, white evangelicals backed Republicans overwhelmingly - by a margin of 70 percent to 28 percent. Depending on the survey and the precise wording of the question, polls typically show that evangelical or "born-again" Christians make up between 30 percent and 40 percent of the U.S. population.

David Barton, a former vice president of the Republican Party of Texas, said, "The key for Mr. McCain is to pick someone who opposes abortion but doesn't alienate any part of the general Republican voting coalition" as Mr. Romney does.

Longtime social-conservative leaders such as Phyllis Schlafly, Phil Burress, Donald P. Hodel and Mathew Staver said earlier this month that they can rally their voters around Mr. McCain largely on the issues of abortion and the judiciary, as long as they are confident that the vice-presidential candidate is pro-life. They are skeptical about Mr. Romney's views.

Mr. Barton, founder of the national pro-life group WallBuilders, said the downside for picking either Mr. Romney or Mr. Huckabee is that evangelicals still would vote for Mr. McCain on Nov. 4 - given the alternative of Mr. Obama - but not work as hard organizing and getting out the vote.

"Romney would bring to the ticket as much enthusiasm from supporters as Huckabee would bring, but Romney's would be from fiscal conservatives and Huckabee's would be evangelicals," he said.

Similarly, a Huckabee choice would leave fiscal conservatives voting for Mr. McCain but otherwise sitting on their hands. Mr. Romney has long been a successful fundraiser - a skill needed because Mr. Obama is expected to raise hundreds of millions of dollars.

Republican strategists close to the Romney camp are promoting the former presidential contender behind the scenes.

"Romney really doesn't think he will be chosen, and that there are far better veep choices for McCain. But in my view, Mitt checks a lot of boxes: He's vetted, he's a Washington outsider, he's conservative, he's a proven vote-getter in Michigan, and he can raise a ton of cash fast for the McCain campaign. He can be the economic voice for the McCain campaign," a conservative Republican strategist close to the Romney organization told The Times.